The N2PK VNA Brian Flynn GM8BJF
Transcription
The N2PK VNA Brian Flynn GM8BJF
The N2PK VNA Brian Flynn GM8BJF What is a VNA? • Measures the transmission and reflection properties of a network. • Technique has been around since the 1970s • Instrumentation is expensive • Normally beyond the scope of amateurs The H-P 8410 (History!) The H-P 8510 (Current) The H-P 8573C (Current) Why reflection and transmission? • The reflection and transmission properties can completely describe a network. • A “network” can be any device • A load is a one port network • Filters or amplifiers are two-port networks • Directional couplers are 3 or 4 port devices A one –port network reflected jX Zo R+jX R incident 1.0 z=r=jx 2.0 0.5 0.2 | | O 0.2 0.5 1.0 oo 0 -0.2 -2.0 -0.5 -1.0 Reflecttion coefficient Smith chart The Smith Chart 90 degrees 1.0 2.0 0.5 inductive reactance 0.2 ρ or Γ short circuit 0 0.2 0.5 1.0 oo open circuit "pure" resistance -0.2 -2.0 -0.5 -1.0 -90 degrees capacitive reactance A two-port network X Z L a1 S11 b1 S12 Zo • • • • Y S21 b2 Z L S22 a2 Z X Y o S11 is complex input reflection coefficient S22 is complex output reflection coefficient S21 is forward transmission coefficient (gain) S12 is reverse transmission coefficient (isolation) Black box representation of circuits • Approach is very versatile • A knowledge of 4 S-parameters completely characterises the network. • The VNA is the means of making these measurements. The N2PK approach to VNA design • Commercial instruments work with a superhet based architecture • N2PK uses a direct conversion approach • Depends on direct digital synthesis of test signals and accurate phase detection N2PK VNA Block diagram PCB layout Back view The Reflection bridge Calibration • The accuracy of measurements depend on comparison with known standards. • Open circuit • Short circuit • 50 ohm load • Through piece Software • A large part of the instrument’s functionality is dependent on the software (which is freely downloadable). • N2PK • W3WWV – EXETER • GM3SEK – VNA4WIN N2PK N2PK Exeter Exeter VNA4WIN VNA4WIN N2PK a la GM8BJF The Reflection Bridge The Calibration Standards Plot of My 7MHz Dipole A Live Demonstration. Further Plans . Convert to USB (Plug n’ Pray) Extend frequency range Fast ADC conversion (allows real-time measurement) Acknowledgements • • • • • Paul Kiciak, N2PK Greg Ordy, W8WWV Ian White, GM3SEK Giancarlo Moda, I7SWX Lou Macknik, W8LI and Suja Raviraj